1. Field of the Invention
The present invention is related to a method for manufacturing gradient index optical elements.
2. Prior Art
Gradient index optical elements have attracted attention because of their excellent aberration correction ability as optics which are essential to the next generation optical systems.
There are various gradient index optical elements which are being researched and developed in many companies and research institutes, including the SELFOC (a registered trademark) lens and slab lens which are already available in the market.
In gradient index optical elements, their medium itself is provided with a power (refractive power) by adding a refractive index distribution. The power depends on the refractive index distribution, and it is only necessary to increase the gradient difference of refractive index n (hereinafter referred to as .DELTA.n) in order to increase the power. Accordingly, at present, to increase the .DELTA.n is a large problem given to the research and development of gradient index optical elements, and many researchers are studying how to increase the .DELTA.n. For instance, for the optics which are marketed under the name of SELFOC lens, the .DELTA.n is increased by providing a concentration gradient of T1 by ion exchange. Also, by providing a concentration gradient of Ag using a double ion exchange method, a lens was obtained for which .DELTA.n.apprxeq.0.1 (the 28th Discussion on Glass).
In addition, by the sol-gel method, a lens for which .DELTA.n=0.04 was obtained by providing concentration gradients of Pb and K (J. Non-cry. sol. 100, 506, 1988), and a lens for which .DELTA.n=0.03 was obtained by providing a concentration gradient of Ti or Ge (Elect. Lett, 22, 99 (1986), Elect. Lett, 22, 1108 (1986)).
Incidentally, the developments of gradient index optical elements up to the present have mainly an approach of increasing the outer diameter, but the measures to decrease the chromatic aberration possessed by the optics themselves have been late. Since refractive index distribution type optics have excellent aberration correction ability, it is possible to drastically decrease the number of the constituent lenses, but there is an inconsistency that the chromatic aberration correction of a lens system becomes more difficult as the number of the lenses decreases. Accordingly, to make a lens system which includes a gradient index optical elements and has the chromatic aberration thereof fully corrected, it may be needed to take measures such as addition of an achromatic lens depending on the case, and thus the merit of the gradient index optical elements is reduced by half.
Thus, to make a lens system in which the number of lenses is small and the chromatic aberration is also corrected, it is important to decrease the chromatic aberration itself generated in each lens and it is important to decrease the chromatic aberration of the gradient index optical elements itself. For this, the following characteristics are desired as the characteristics required for the medium of gradient index optical elements.
In radial gradient index optical elements, the refractive index of the medium differs depending on the position through which a light beam is passing (the distance from the axis), and hence the refractive index of the light beam differs. If it is now supposed that the Abbe number ##EQU1## of the medium is constant, then in a portion having a high refractive index, the light beam as it passes through the medium largely refracts as shown in FIG. 1 (a), so that the spread due to the difference in the wavelength of the light beam becomes large as compared with a portion having a low refractive index. That is, if the Abbe number .nu..sub.d is fixed, the chromatic aberration (n.sub.F -n.sub.C) increases as the refractive index n.sub.d increases. Consequently, in order to decrease the chromatic aberration (n.sub.F -n.sub.C), it is desirable that the Abbe number .nu..sub.d is large in the portion having a high refractive index as shown in FIG. 1 (b). That is, the characteristics changing in the form of high refractive index.multidot.low dispersion-low refractive index.multidot.high dispersion are desirable as the characteristics of the medium.
Also, axial gradient index optical elements can be considered similarly to the conclusion obtained for the conventional achromatic junction lens (tablet) shown in FIGS. 2 (a) and (b). Thus, in axial gradient index optical elements, the junction of a high-refractive index medium lens with a low-refractive index medium lens is achieved by providing the medium with a refractive index distribution within the same lens as shown in FIGS. 2 (c) and (d), and it is therefore desirable for the characteristics to vary in the form of high refractive index.multidot.low dispersion-low refractive index.multidot.high dispersion similarly to radial gradient index optical elements. This indicates that optics whose optical characteristics change in the direction A on the n.sub.d -.nu..sub.d graph shown in FIG. 3 are more excellent in the point of chromatic aberration correction than those having optical characteristics that change in the direction B (refer to the Japanese Patent Application Laid-Open No. 218614/1985 official gazette).
However, few of the gradient index optical elements which are presently being developed have such characteristics and even in those which have already been actualized, .DELTA.n remains to be a very small value.
That is mainly caused by the manufacturing method of gradient index optical elements. For instance, in the ion exchange method, in order to make .DELTA.n large, a concentration gradient is provided by the ion exchange between Tl.sup.+ ions having a valence of one, which are introduced into glass so as to constitute a glass modification oxide (that is not directly related to the glass formation), and N.sup.+ or K.sup.+ ions. But, the use of Tl.sup.+ makes .DELTA.n large whereas the change characteristics of the Abbe number becomes a high refractive index.multidot.high dispersion-low refractive index.multidot.low dispersion type, and thus a chromatic aberration will largely occur. Also, .DELTA.n can be made large by the exchange of Ag.sup.+ with Na.sup.+, but a large chromatic aberration will similarly occur.
In addition, there is an instance in which the chromatic aberration is significantly improved using Li.sup.+, but on the other hand, .DELTA.n becomes small and its effect is not fully exhibited. Although .DELTA.n can be improved by increasing the content of Li.sup.+, there is a limit because of the resistance of the glass body material and the difficulty of the technique of stably dissolving the easily volatile alkali content into the glass body material, and the one of a level which actually exhibits a sufficient effect has not yet been obtained.
Since, in the ion exchange method, a concentration gradient can essentially be provided only by positive ions having a valence of one because the exchange speed of ions having a valence larger than one is extremely low, the variations of the ion concentration gradient for providing the distribution thereof are very limited, and thus the one has not yet been achieved in which .DELTA.n is large and the occurrence of a chromatic aberration is low as described above.
Further, the development by the sol-gel method is now proceeded with, and there is a method wherein a metal element such as Ti, Ge or Zr which enhances the refractive index and constitutes the glass forming oxide (that is originally contained for forming glass) is eluted from a wet gel by an acid or the like. In this method, the change characteristics of the Abbe number are of the high refractive index.multidot.high dispersion-low refractive index.multidot.low dispersion type though a .DELTA.n which is large to some extent is obtained, and thus a chromatic aberration largely occurs and the characteristics of the gradient index optical elements are near to those obtained by the ion exchange of the Tl.sup.+ - Na.sup.+ type.
Also, in the Japanese Patent Publication No. 15492/1987 official gazette, it is shown that chromatic aberration (dispersion) can be made small using Nb, Ta, Sc, Y, La or Th as a dopant metal. However, this method has no difference in that dispersion changes from a high dispersion to a low dispersion as the refractive index changes from a high refractive index to a low refractive index, and the amount of change in its dispersion value is merely small. That is, since the dispersion is eventually shown by the reciprocal of the Abbe number, the method described in the Japanese Patent Publication No. 15492/1987 official gazette has no difference in that it is the dispersion of the direction B in FIG. 3 of the specification of this application, and its angle is merely made such that the arrow in the figure stands.
Moreover, in the Japanese Patent Publication No. 6295/1985 official gazette, a multiple-dope method is disclosed in which a molecular stuffing method is repeated. But, this method had two problems that a .DELTA.n which can practically be used is not obtained because of the small amount of a dopant that can be doped at a time, and that it relies on an impractical process in which a baking at a high temperature in a furnace is performed for each doping.
The present invention was made in view of such prior art problems, and its object is to provide a method for manufacturing gradient index optical elements having various characteristics, including gradient index optical elements having an excellent chromatic aberration correction ability, in which .DELTA.n is large enough for practical use and the characteristics change of the Abbe number is of a high refractive index.multidot.low dispersion-low refractive index.multidot.high dispersion type.